Cell lines showing partial or total loss of their transformed characteristics have been isolated from rodent cell cultures transformed by SV40, polyoma virus and Rous sarcoma virus. The in vitro growth behaviour and surface properties of these revertant cell lines are under investigation, together with studies on their in vivo tumorigenicity. Somatic cell hybrids produced by fusion of revertant cell lines with various normal and transformed cells have also been isolated and are being used to study the stability and expression of the transformed phenotype in the presence of a normal or revertant cell genome. Observations on the karyotypic properties of transformed cells, revertants and hybrids between revertant and transformed cells during serial passage in vivo are in progress.